1. Field of the Invention composed of a luminance signal and a chrominance signal.
2. Description of the Related Art
An electronic still video system has been known as one of the image signal processing apparatuses of the kind arranged to process an image signal composed of luminance and chrominance signals. The electronic still video system is arranged to record a still image signal for one field or one frame in circular tracks which are concentrically formed on a magnetic disc called a video floppy disc. The still image signal for one field is recorded in one track. The still image signal for one frame is recorded consecutively in two tracks on the video floppy disc. The still video system is further arranged to reproduce the image signal recorded on the video floppy disc and either to display the reproduced still image on a TV monitor device or to have it printed in a hard copy by a printer device.
In recording the image signal composed of luminance and chrominance signals, the luminance signal is frequency-modulated into a high-frequency band. As for the chrominance signal, two kinds of color-difference signals R-Y and B-Y are converted into a line-sequential color-difference signal. After that, the line-sequential color-difference signal is frequency-modulated into a low-frequency band. The frequency-modulated luminance signal and the frequency-modulated line-sequential color-difference signal are frequency-multiplexed into a recording image signal to be recorded on the video floppy disc. In reproducing the image signal recorded, the signal reproduced from the video floppy disc is processed to extract the frequency-modulated luminance signal and the frequency-modulated line-sequential color-difference signal. The extracted signals are frequency-demodulated. After the frequency demodulation, the line-sequential color-difference signal is subjected to a simultaneous rearrangement process for converting it into two simultaneous color-difference signals. After that, the two color-difference signals are converted into a chrominance signal through a quadrature two-phase modulation process. Then, the frequency-demodulated luminance signal and the chrominance signal thus obtained are frequency-multiplexed into a video signal which conforms, for example, to the NTSC television system. The video signal thus obtained is supplied to a TV monitor device or to a printer device.
The electronic still video system has a mode called a field reproduction mode. In the field reproduction mode, an image signal for one field recorded in one track on the video floppy disc is reproduced, then an image signal for one frame, i.e., for two fields, is formed from the reproduced image signal for one field, and the image signal for one frame thus formed is supplied to a TV monitor device or to a printer device. In the case of the field reproduction mode, the image signal for one frame composed of two field signals includes an image signal for one field which corresponds to the image signal for one field reproduced from the video floppy disc (hereinafter referred to as a reproduced field image signal) and an image signal for the other field which is formed from the reproduced field image signal (hereinafter referred to as an interpolation field image signal). In a case where the reproduced field image signal obtained from the video floppy disc is displayed on a TV monitor device in forming the interpolation field image signal, the interpolation field image signal is formed by a process called a field interpolation process by taking the arithmetic mean of image signals corresponding to a plurality of horizontal scanning lines which adjacently appear on the display image plane of the monitor device. Further, a process called a skew correction process is carried out for correcting a time deviation of 1/2 H (H: a horizontal scanning period) between the image signal of a field which consists of odd-number-th horizontal scanning lines and the image signal of a field which consists of even-number-th horizontal scanning lines, which are formed by the field interpolation process.
Further, the electronic still video system is arranged to carry out a process called a dropout compensation process to make compensation for a signal dropout if any signal dropout takes place in reproducing the image signal recorded on the video floppy disc.
In carrying out the simultaneous rearrangement process for the line-sequential color-difference signal, the dropout compensation process at the time of reproduction and also the field interpolation process and the skew correction processes in the case of field reproduction mode, it is necessary for the electronic still video system to delay the image signal as much as one horizontal scanning period. For this purpose, delay elements of varied kinds, such as a glass delay line, a CCD (charge coupled device), etc., have been employed by the conventional still video system.
Meanwhile, an electronic still video system of the following kind has recently come to be considered: This system includes a DRAM (dynamic random access memory) which is capable of storing an image signal for one field or one frame and permits a random access to any desired digital image signal stored by designating the address of the digital image signal. At the time of reproduction, the video floppy disc is driven to rotate by a motor. An image signal for one field (or for one frame) reproduced is digitized. The digital image signal is stored in the DRAM. After that, the rotative driving action of the motor is stopped. The digital image signal stored in the DRAM is repeatedly read out for every one field (or frame) period. The digital image signal read out is converted into an analog signal and outputted. This arrangement effectively lowers the power consumption of the reproducing operation of the system.
Another advantage of the electronic still video system having the DRAM lies in that: After the image signal reproduced from the video floppy disc is stored in the DRAM at the time of reproduction, any desired image signal is accessible by address designation as mentioned above. The use of the DRAM enables the still video system to carry out, without recourse to the above-stated various delay elements such as a glass delay line or a CCD, the simultaneous rearrangement process on the line-sequential color-difference signal and the dropout compensation process which are necessary for reproduction and the field interpolation process and the skew correction process which are necessary in the field reproduction mode.
More specifically, with respect to the dropout compensation process, for example, in reading the image signal stored in the DRAM, an image signal of an address for a horizontal scanning period which is one horizontal scanning period before or after an address corresponding to a period during which a dropout takes place during the process of reproduction is read out in place of the dropout. As to the simultaneous rearrangement process on the line-sequential color-difference signal, the process can be carried out by simultaneously reading out the signals of two different addresses deviating as much as one horizontal scanning period from each other in reading out the line-sequential color-difference signal stored in the DRAM.
However, the DRAM which is arranged to be capable of storing a digitized image signal for one field (or for one frame) and to permit a random access to a desired digital image signal by designating the address of the digital image signal stored necessitates signal writing or reading to be carried out at a high speed, whereas a DRAM for universal purposes permits signal writing or reading only at a slow speed. In order to carry out signal writing or reading at a high speed, the signal writing or reading must be carried out by parallel processing. The parallel processing necessitates use of a plurality of DRAMs, which makes the structural arrangement of the system complex and thus makes reduction in size and weight of the system difficult.
Further, the electronic still video system may be arranged to use a dual port DRAM which permits signal writing or reading at a high speed. However, the dual port DRAM is very expensive. The use of the dual port DRAM, therefore, causes an increase in cost.